1. Field of the Invention
This invention relates to current dividers used in conjunction with a current sensing transducer of an electric energy measuring meter. More particularly, this invention relates to those current dividers which provide a fixed and predetermined fractional value of a current to be sampled independently of the ratio of the cross-sectional areas of the current-carrying members.
2. Description of the Prior Art
It is desirable in countless applications to sample the current flowing in a conductor. One such application is the measurement of electrical energy quantities such as kilowatts, kilowatt hours, volt ampere hours, etc.
In typical electrical energy measurements at an electric utility customer location, line current varies typically from one-half ampere to 200 amperes or in a current variation ratio of approximately 1 to 400. Current instrument transformers that are required to produce low level output signals from the aforementioned widely-varying current input often require structural arrangements which are of substantial size and cost. This is due to the fact that in a current transformer, the ampere turns of the primary must equal the ampere turns of the secondary. Since maximum current levels of the primary may reach 200 amperes, the secondary winding size becomes substantial in order to produce linear low level output signals. One solution to this problem is to use a current sensing transducer in place of a current transformer.
One example of a current-sensing transducer is disclosed in U.S. Patent Application Ser. No. 923,620 for a "Current Sensing Transducer For Power Line Current Measurements", which is assigned to the assignee of this invention. The current-sensing transducer includes a conductive current divider having a branch path for conducting a fractional current proportional to the current to be sensed. This branch path is used as the primary winding of a single turn compensated transformer having two secondary windings. An amplifier circuit controls the current input to one of the secondary windings in response to the output of the other of the secondary windings such that the flux in the transformer core due to the branch current is virtually compensated. The secondary winding current is a measure of the current flowing in the branch path. Since the current flowing in the branch path is a fixed proportion of the current to be sensed, the secondary winding current is a measure of the current to be sensed.
It is apparent from the foregoing that current-sensing transducers operated in conjunction with a current divider are of substantially less size and cost than a conventional current transformer. A typical current divider which may be used with a current sensing transducer is disclosed in U.S. Pat. No. 2,915,707 for a "Current Measuring Reactor Arrangement" which discloses a current divider that is insensitive to ambient electromagnetic fields. A conductor is provided with an aperture and a conductive shunt of reduced cross-sectional area. It is an inherent feature that the current flowing in the conductive shunt is a ratio of the cross-sectional area of the conductive shunt and the cross-sectional area of the other conductive paths.
As seen from the above example current dividers typically utilize the principle that a fractional current may be produced by providing two or more current paths having a ratio of cross-sectional areas, or inverse lengths, which are the same fraction as that desired for the sample current. For example, if a current in a branch path is to be 1/40 of the current in a major path, the cross-sectional area, or inverse length, of the branch path must be 1/40 of the cross-sectional area, or inverse length, of the major path. However, problems are encountered when extremely small currents are to be split off from a substantially large current or if the overall physical size of the current divider is to be extremely small. In these cases, by providing the proper ratio of cross-sectional areas, or inverse lengths, the shunting or branch path becomes so small that it is no longer structurally sound. In addition, small branch paths may become subject to self-heating causing thermal gradients thereby destroying the proportionality of the current divider.